Legendary Hacker in the City

In this story computer security is a business, and a fight over hacking techniques can pull in a whole company. One of the companies caught in such a fight is a network security firm run by a woman chief executive. John Mercer had been the best hacker in the country before he retired. He set up a shop repairing ordinary computers, and the people who come in know him only as Mr. Mercer. A chance encounter one night with the woman who runs the security firm pulls him into the dispute her company is caught in. Each time she is backed into a corner he steps in with his own skills and gets her out. Along the way he enters a hacking contest and a programming contest, and the team he is on takes home millions.